Is there a Java equivalent of Python's defaultdict?

Question

In Python, the defaultdict class provides a convenient way to create a mapping from key -> [list of values], in the following example,

from collections import defaultdict d = defaultdict(list) d[1].append(2) d[1].append(3) # d is now {1: [2, 3]} 

Is there an equivalent to this in Java?

Answer 1

There is nothing that gives the behaviour of default dict out of the box. However creating your own default dict in Java would not be that difficult.

import java.util.ArrayList; import java.util.HashMap; import java.util.List;  public class DefaultDict<K, V> extends HashMap<K, V> {      Class<V> klass;     public DefaultDict(Class klass) {         this.klass = klass;         }      @Override     public V get(Object key) {         V returnValue = super.get(key);         if (returnValue == null) {             try {                 returnValue = klass.newInstance();             } catch (Exception e) {                 throw new RuntimeException(e);             }             this.put((K) key, returnValue);         }         return returnValue;     }     } 

This class could be used like below:

public static void main(String[] args) {     DefaultDict<Integer, List<Integer>> dict =         new DefaultDict<Integer, List<Integer>>(ArrayList.class);     dict.get(1).add(2);     dict.get(1).add(3);     System.out.println(dict); } 

This code would print: {1=[2, 3]}